MRAM is a nonvolatile memory technology that uses magnetization to represent stored data, in contrast to older RAM technologies that use electronic charges to store data. One primary benefit of MRAM is that it retains the stored data in the absence of electricity, i.e., it is a nonvolatile memory. Generally, MRAM includes a large number of magnetic cells formed on a semiconductor substrate, where each cell represents one data bit. A bit is written to a cell by changing the magnetization direction of a magnetic element within the cell, and a bit is read by measuring the resistance of the cell (low resistance typically represents a “0” bit and high resistance typically represents a “1” bit).
A practical MRAM device will typically include millions of cells. Generally, a single MRAM cell includes an upper ferromagnetic layer, a lower ferromagnetic layer, and an insulating layer between the two ferromagnetic layers. The upper ferromagnetic layer is the free magnetic layer because the direction of its magnetization can be switched to change the bit status of the cell. The lower ferromagnetic layer is the fixed magnetic layer because the direction of its magnetization does not change. When the magnetization in the upper ferromagnetic layer is parallel to the magnetization in the lower ferromagnetic layer, the resistance across the cell is relatively low. When the magnetization in the upper ferromagnetic layer is anti-parallel to the magnetization in the lower ferromagnetic layer, the resistance across the cell is relatively high. The data (“0” or “1”) in a given cell is read by measuring the resistance of the cell. In this regard, electrical conductors attached to the cells are utilized to read the MRAM data.
The orientation of magnetization in the free magnetic layer can point in one of two opposite directions, while the orientation of the fixed magnetic layer can only point in one direction. In conventional MRAM, the orientation of the magnetization in the free magnetic layer rotates in response to current flowing in a digit line and in response to current flowing in a write line. Selecting the directions of the currents will cause the magnetization in the free magnetic layer to switch from parallel to anti-parallel to the magnetization in the fixed magnetic layer. In a typical MRAM, the orientation of the bit is switched by reversing the polarity of the current in the write line while keeping a constant polarity of the current in the digit line.
The traditional MRAM switching technique has some practical limitations, particularly when the design calls for scaling the bit cell to smaller dimensions. For example, since this technique requires two sets of magnetic field write lines, the array of MRAM cells is susceptible to bit disturbs (i.e., neighboring cells may be unintentionally altered in response to the write current directed to a given cell). Furthermore, decreasing the physical size of the MRAM free layer results in lower magnetic stability against magnetization switching due to thermal fluctuations. The stability of the bit can be enhanced by utilizing a magnetic material for the free layer with a large magnetic anisotropy and therefore a large switching field, but then the currents required to generate a magnetic field strong enough to switch the bit are impractical in real applications.
Transmission mode spin-transfer switching is another technique for writing MRAM bit data. Writing bits using the spin-transfer interaction can be desirable because bits with a large coercivity (Hc) in terms of magnetic field induced switching (close to 1000 Oersteds (Oe)) can be switched using only a modest current, e.g., less than 5 mA. The higher Hc results in greater thermal stability and less possibility for disturbs. A conventional transmission mode spin-transfer switching technique for an MRAM cell includes a first magnetic layer, a nonmagnetic tunnel barrier layer, and a second magnetic layer. In this technique, the write current actually flows through the tunnel junction in the cell. According to the spin-transfer effect, the electrons in the write current become spin-polarized after they pass through the fixed magnetic layer. In this regard, the fixed layer functions as a polarizer. The spin-polarized electrons cross the nonmagnetic layer and, through conservation of angular momentum, impart a torque on free magnetic layer. This torque causes the orientation of magnetization in the free magnetic layer to be parallel to the orientation of magnetization in the fixed magnetic layer. The parallel magnetizations will remain stable until a write current of opposite direction switches the orientation of magnetization in the free magnetic layer to be anti-parallel to the orientation of magnetization in the fixed magnetic layer.
The transmission mode spin-transfer switching technique requires relatively low power (compared to the conventional switching technique), virtually eliminates the problem of bit disturbs, results in improved data retention, and is desirable for small scale applications. In practice, however, this technique is difficult to implement in a memory array because the write current must flow through the magnetic tunnel junction embodied in the cell. This negatively affects the reliability of the MRAM cells and requires the use of larger write transistors at each bit location that are capable of producing the necessary currents, which is incompatible with high-density applications.
As disclosed in U.S. Patent Publication No. 2006/0087880 A1, a magnetic random access memory (“MRAM”) device can be selectively written using spin-transfer reflection mode techniques. Selectivity of a designated MRAM cell within an MRAM array is achieved by the dependence of the spin-transfer switching current on the relative angle between the magnetizations of the polarizer element and the free magnetic element in the MRAM cell. The polarizer element has a variable magnetization that can be altered in response to the application of a current, e.g., a digit line current. When the magnetization of the polarizer element is in the natural default orientation, the data in the MRAM cell is preserved. When the magnetization of the polarizer element is switched, the data in the MRAM cell can be written in response to the application of a relatively low write current.
A technique is disclosed in U.S. Pat. No. 6,967,863 to reduce the critical current density for spin transfer by using a free layer with perpendicular anisotropy and out of plane magnetization. Another technique is disclosed in WIPO publication WO 2005/082061 A2 to reduce the critical current density for spin transfer by using a free layer with perpendicular anisotropy and in-plane magnetization.
However, these methods require specific materials for the free layer, which can negatively affect switching reliability and also the read signal.
Accordingly, it is desirable to provide a spin torque MRAM cell having a reduced switching current wherein standard materials may be used for the free layer. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.